For the aforesaid purposes, electrostatic dust separators generally are used, in which the dust particles are electrically charged and caused to deposit on precipitation electrodes.
One such known apparatus comprises a plurality of great plates, i.e. large metal sheets or plates made of a sheet metal material and having a square or rectangular shape, spaced from each other. Between the plates, frames are located in which a number of wires are clamped. A high-voltage rectifier, usually of 50 kW, is connected with its negative end to the wires, while its other end is grounded and connected to the plates. In this way, a strong electric field between the wires in the framework and the plate curtains (plates as described above which hang like curtains) is obtained. The field intensity is highest adjacent the wire surfaces where it becomes so high that electrical discharge in the form of corona discharge occurs along the wires. At the ionization of the gas effected about the wires, great amounts of positive and negative ions are formed continuously. The positive ions immediately are attracted to the negative wires. The negative ions, however, move from the respective wire to the nearest located plate curtain. The gas, from which the dust or particles are to be separated, is directed between the plate curtains.
On their path to the plate curtains, the ions to some extent colloid with and adhere on particles contained in the gas flow. The particles are therefore also negatively charged and start moving in the same direction as the ions, i.e. to the nearest located plate curtain. Upon contact with the plate curtains, the particles get deposited as a coat thereon and are discharged.
A certain dust coating takes place also on the wires, due to the fact that particles passing in the vicinity thereof can be met by positive ions found there and be attracted to the electrodes. The dust deposition caused in this way is quantitatively relatively small, but unfavourable for the corona discharge.
Increasing dust coat on the curtain plates and emission electrodes weakens the electric field and increases the ohmic resistance between the plates and electrodes. Both of these conditions reduce the dust separating capacity of the apparatus, and, therefore, the dust deposited must be removed step by step. This is carried out intermittently by causing the plates and frames to vibrate, so-called "pounding", by means of mechanical devices, at certain times. The agglomerated dust coat thereby gets loose and drops down into a dust collecting bin provided below.
The electrostatic filters presently in use and described above are bulky and expensive to install and operate. Moreover, they have other disadvantages which substantially reduce their dust separating capacity.
The aforesaid apparatuses, among others, are sensitive to variations in operation which give rise to more considerable variations in the dust concentration in the gas. For example, if for some reason the dust concentration happens to rise substantially, the dust coat on the plate curtains rapidly increases and increases also on the emission electrodes. As a result, the electric field is weakened and the electron emission is hampered. Therefore, a substantial portion of the charged dust particles do not have time to move all the way to a plate curtain, and instead follow along with the gas flow out of the apparatus. Consequently, they are not separated by the apparatus.
In the prior art, pounding of plate curtains and frames is started and stopped by timer and, thus, occurs without being related to the variations in the dust concentration in the gas, as should be required.
It is also quite common in prior art apparatuses for portions of the dust to get loose spontaneously when the dust deposition on the plate curtains increases in thickness. The apparatus is designed in such a way that all dust disengaging from the electrodes, either spontaneously or caused by said pounding, drops down in free fall to the dust collecting bin. During the fall, a great amount of the dust portions burst and are disseminated into a dust cloud which follows along with the flowing gas out of the apparatus, unless the fan means have been switched off. It is known by experience that the fan means, which in view of the aforesaid must have been switched off during the "pounding" in order to prevent great amounts of dust from being emitted into the ambient air, in many cases are switched off for too short a time or not at all.
When the electric field is increased in order to increase the dust retaining capacity of the plate curtains, glow discharge can occur in the dust layer. In that case, correspondingly to corona discharge along the emission electrodes, great amounts of both positive and negative ions are developed in the dust layer. The negative ions immediately drop in against the plate curtains. The positive ions, however, are drawn out into the space between the plate curtains and migrate to the wires. In the space between the wires and plate curtains both negative and positive ions will then be found and neutralize each other. This charge of the dust particles disappears entirely or partially, and the dust is moved out of the apparatus together with the gas flow.
By using a dust separation apparatus according to the present invention, inconveniences of the kind involved with the aforesaid apparatuses and their deteriorating dust separation capacity do not arise.